1. Field of the Invention
The present invention relates to an information recording method, an information recording apparatus and an optical disk, for recording record marks corresponding to multilevel data on an optical disk such as a rewriteable phase change type optical disk, a program for causing a computer to execute the information recording method, and a computer readable recording medium recoding the program.
2. Description of the Related Art
Recently, along with progress of digital technology and improvement of data compression technology, an optical disk such as a CD-ROM, or a DVD-ROM having the same diameter but being able to record approximately seven times of data, has become taking attention. An optical disk apparatus reproducing contents from the optical disk has spread with the costs thereof reduced.
The optical disk only for reproduction such as a CD-ROM or a DVD-ROM has, on its recording surface, spiral or concentric pit series produced thereon. Information is recorded with the use of a length of a pit, an interval therebetween and a combination thereof. In this case, information is transformed (binarized) into a combination of two numerals (binary), i.e., 0 and 1, and is written in the optical disk. Such a recording way is called a binary recording method.
An information size of the above-mentioned contents increases year by year, and a further increase in the information size recordable in the optical disk is expected. As a way of increasing the information size recordable to the optical disk, a way of writing in the optical disk in such a manner that the information is transformed into more than the two of numerals. Various types of related arts have been developed eagerly for taking this art into a practical use. Below, for the sake of convenience, to transform information into more than the two numerals is referred to as, to transform into multilevel data. Further, such a way of recording information with multilevel data is referred to as a multilevel recording method.
As to rewriteable optical disk recording, individual difference and/or operation environment of the optical disk and the optical disk apparatus have an influence. Therefore, recording conditions should be optimized for each combination thereof. For example, for a case of a phase change type optical disk, in order to produce desired record marks, a recording laser light emitting time width, recording power and erasure power are controlled.
According to Patent Document 1 for multilevel data recording, in order to positively reproduce multilevel-recorded data, trial writing is carried out, and test recording is carried out until a desired reproduced signal is obtained. Recording correction (recording condition modification) is carried out by the following process:
(1) Test data is recorded/reproduced;
(2) An ideal waveform and a reproduced signal waveform are compared;
(3) Convergence occurs? If Yes, the process is finished;
(4) If No, the laser emitting condition is corrected; and
(5) The process is returned to (1).
With reference to FIG. 12, a multilevel recording method to which the present invention is applicable is described. Recording is carried out for each recording cell having a fixed length on a recording track, in such a manner that a reproduced signal level changes corresponding to multilevel data. Reproduction of multilevel data is carried out such that the reproduced signal is sampled at a predetermined frequency (for example, at a central position of the recording cell), and the multilevel data is determined from the sampled reflected light intensity. A reproduction light spot diameter is longer than a circumferential direction length of the recording cell, and thus, intersymbol interference occurs. In the prior art, in consideration of the intersymbol inference, the above-mentioned recording correction is carried out. Thereby, the multilevel data can be determined precisely. For 8-level recording, combinations of multilevel data to consider amount to 83=512. Accordingly, a considerable time is required for optimization of the recording conditions.
Japanese Laid-open Patent Application No. 2003-091822 discloses a recording method by which, even with an easy method without carrying out complicated process of Japanese Laid-open Patent Application No. 10-134353, the optimum recording conditions (recording power and recording pulse widths) can be determined. According to Japanese Laid-open Patent Application No. 2003-091822, recording power by which reflected light intensity is saturated is obtained as optimum recording power, while a pulse width by which reflected light intensity is saturated is obtained as an optimum pulse width. Then, based on the optimum pulse width, other multilevel data pulse widths are determined. The pulse width proposed there is defined as a ratio between a total of a top pulse width Ton and an off pulse width Toff, and the cell time width Tc (see FIG. 1).    Patent Document 1: Japanese Laid-open Patent Application No. 10-134353; and    Patent Document 2: Japanese Laid-open Patent Application No. 2003-091822